Terminal bushings



c. F. sNNi-:N BERG 3,001,005

TERMINAL BUSHINGS sept. 19, 1961 Filed July 25, 1959 Fig.5

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Fig. 3

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United States Patent O 3,001,005 TERMINAL BUSHINGS 'Charles F. Sonnenberg, Penn Hills Township, Allegheny County, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed July 23, 1959, Ser. No. 829,091 Claims. (Cl. 174-142) This invention relates, generally, to terminal or entrance bushings and, more particularly to bushings of cast resin.

The fundamental purpose of an entrance bushing in electrical apparatus is to allow passage of a current conductor at some voltage above ground through a grounded tank or container. Since the tank'top is usually a relatively thin sheet of steel, the voltage stress is concen trated on a small section of the bushing surface, much like a razor edge attempting to cut it.

Bushings with metal foils were developed utilizing a condenser effect, and foilless bushings have been provided with a metallic mounting lia-nge brazed to a metallic cylinderwhich was pressed onto the bushing. Both of these methods have satisfactorily distributed the voltage stress over a greater surface area.

Terminalbushings of cast epoxy resin have been considered by apparatus manufacturers, both domestic and foreign, for several years. The method most often advanced has been to mold the bushing in a single casting 4to include the condenser, weather casing and mounting flange. However, there are certain objections to a bushing constructed in the foregoing manner, particularly fwhen it is to be utilized in outdoor service. The surface is subjected to erosion and itis diiiicult to produce a cast surface which is as glossy and smooth, and, thus,

resin bushing having an integrally bonded metallic mount- -i ng ilange and metallic cylinder.

IA further object of the inventionY isto provide a cast fresin bushing having a metallic cylinder which is utilized as an outer ground foil and as a portion of the mold for lcastin g the resin.

Still another object of the invention is to provide a cast resin bushing having a metallic screen embedded Within the molded material.

A further object of the invention is to utilize a porcelain casing as part of the mold for a cast resin bushing.

-A still further object of the invention is to assure a voi'dless bond between the porcelain casing and the molded portion of a bushing.

Other objects of the invention will be explainedrfully hereinafter or will be apparent to those skilled in the art.

` I n accordance with one embodiment of the invention, a

cast-y resin bushing is formed by securing one end of a cylindrical metal sleeve to one side of a mounting flange and utilizing the sleeve as part of a mold for casting the bushing. The inside of a porcelain casing is lined with a-exible, elastic material and the casing is disposed on `the other side of the mounting flange and utilized as another part o-f the mold. A removable mold is provided at the other end of the metal sleeve. Thus, the casing and the metal sleeve are bonded to the cast resin to form the bushing.

For a better understanding of the nature and objects of the invention, reference may be had to the following 'detailed description, 'taken in conjunction with the ac- 'companying drawing, in which:

FIGURE 1 is. a view, in section, of a cast bushing embodying features of the invention with the casting ,molds in position; l

FIG. 2 is a view, partly in section and partly in ele vation, of the bushing shown in FIG. 1, the molds being removed;

FIGS. 3 and 4 are views, similar to FIGS. l and 2, respectively, of another embodiment of the invention, and

FIG. 5 is a view, partly in section and partly in elevation of still another embodiment of the invention.

Referring to the drawing, and particularly to FIGS. l and 2, the terminal bushing shown therein comprises a cylindrical metal sleeve 10, which is secured to a metal mounting flange 11, a conductor 12, which extends longitudinally through the sleeve 10 and is spaced from the sleeve, a thermoset resin body 13 and a cylindrical metal screen 14 which is embedded in the resin body 13 concentrically vwith the conductor 12. The sleeve 10 and mounting flange 11 are preferablyY composed of aluminum since aluminum has substantially the same thermal coecient of expansion vas the thermoset resin material utilized in casting the body. If a resin having a different coefficient of expansion is utilized, the sleeve 10 should be composed of a metal, such as copper, having substantially the same coeliicient of expansion. As shown, the sleeve 10 is welded to the flange 11. However, the sleeve and the flange may be formed as one piece.

A removable mold 15 is provided at the lower end of the sleeve 10. Another removable mold 16 is provided on the upper surface of the flange 11. The mold 16 may be retained in position by attaching a cap 17 to the upper end of the conductor 12.

The thermoset resin utilized for the body 13 should be one that has good physical strength, has a relatively low shrinkage during curing and has good adherence to metal and ceramics, such as porcelain and glass. The resinous polymeric expoxides, such as a thermosetting resin comprising a glycidyl polyether of a polyhydric phenol having a 1,2-epoxy equivalence of between l and 2, have been found to meet the foregoing requirements and have been successfully utilized in making bushings of the present type. Such epoxideresins are described in U.S. Patents 2,728,744 and 2,739,134.

As previously explained, the metal sleeve 10 and the grounding iiange 11 are provided to distribute the voltage stress over a greater surface area. In the present structure, the sleeve 10 is utilized as part of the mold for casting the resin. As described in a copending application of R. G. Black, Serial No. 828,991, liled July 23, 1959, the sleeve 10 may be provided with a plurality of longitudinal slots 18 as shown in FIG. 2. The slots 18 permit the adhesion of the resin to the metal to pull the segments ofthe metal shell inwardly during the curing process, thereby preventing shrinkage of the resin from forming voids between the metal shell and the resin. When the bushing is utilized for` relatively high voltage service, any voids between the resin and the metal may give rise to the phenomenon of internal corona causing radio interference. When the slots 18 are provided in the metal sleeve 10 a rubber sleeve or other temporary means, such as aluminum foil, may bev provided around the sleeve 10 to retain the resin within the sleeve during the casting process.

When the metal sleeve 14 is embedded in the resin body, as shown in FIGS. 1 and 2, the outer cylinder 10 functions as an outer foil, thereby obtaining a condenser effect similar to that obtained in other condenser bushings. The sleeve 14 is preferably an aluminum screen. A screen is utilized instead of foil because it has sulicient strength to support its own weight and thus stand without collapsing in the resin while it is curing. Also, the many perforations in the screen permit the resin to bond completely through the screen. Thus, anyrelaxing caused by 'didcrence :in "thermal coeicience of expansion will not `permit the resin body to relax itself from the screen .and form internal voids. As previously stated, aluminum has substantially the same thermal coeiiicientv of expansion as zthe thermoset resin utilized in the Apresent bushing.

; v While the-resin may bevpoured into 'either one of the molds, the molds are preferably turned upside down and -the resin poured into the mold 15'during the casting proc- After the resin is cured, the `molds 15 and lo Ymay be removed, thereby providing the bushing, shownin FlG. This bushing is suitable for indoor'service. Thei'lange .choices 11 maybe attached to a tank or container for electrical The :casing 21 may beretained in Vposition on the flange 11 by a terminal cap 23 which is attached to the upper end of the :conductor 12. As previouslyy explained, the Isleeve'lt) is preferably provided with a plurality of longitudinal slots 18 and a temporary means is provided on the sleeve to prevent theresin from coming out through the slots 18 during the casting process. After the resin lis cured the mold may be removed, thereby providing the bushing'shown in FlG. 4 having ythe casing21 and the sleeve 10 bonded to the resin body. The electrical connections may be made to the lower end of the conductor Y A212 and to .the terminal cap 2.3. The mounting flange 11 may be attached to the cover of ka tank or container in the l'manner previously4 described.

.lf itis not desired to utilize the casing 21 as part of the moldduring the casting process, the Vresin bodymay be cast yin the manner previously described with refer- .'enceto FIGS. Vl and 2. The elastic liner 22 may be prelining 22 may be a silicone Vrubber or ayneoprene-base --rubber cement. Y 1 v forrned into a double flanged frustrum of a right circular f cone. The inside diameter of the cone would be slightly less than the outside diameter of the cast resin body por- 'ation'fof the bushing and the outside diameter would be .slightly greater than the inside diameter of the porcelain Y casing. The elastic cone would be pressed over the mold- Ped bushing with the lower flanged surface acting as a gasvhetbetween'the porcelaincasing and the mounting flange.

After theV porcelain is. firmly pressed into place over the felastio cone the terminal cap is screwed down providing the compressive force .necessary to keep the porcelain Vin place. The upper'flangecl surface on the elastic cone acts as a gasket between the terminal cap andthe Porcelain.

4ln the modification ofthe invention shown in FIG. 5, la mounting vllange ll is `formed as an integral part of the cast resin bushing body 13'.' The ilange 11 is "attached to a tank cover or wall 24 by means of bolts 2t).

In this manner it is not necessary to provide a metal mounting ange as in the bushings previously described. In order to distribute theY voltage stress over a greater f'area,'a metal screen or disc 25 is embedded in the iiange f V11. -fs'ertsp26 disposedV inthe holes for the mounting bolts 2t). v'The disc 25 is connected to a cylindrical metal screen 27 The disc 25 is connected to metal washers or inwhich is embedded in the resin body i3 con'centrically 'with the 'conductor l2.' Thus, when the bushing is attaclried to a grounded tank the screen 27 is grounded through kthe disc 25, Athevvashers 26 and the mounting bolts 2li, thereby distributing the voltage stress.

It .should be noted that the metal sleeve iti is not at-V tached to the tank cover 2d since the sleeve it) is a part `of the bushing'structureV and is lremovable with the bushi'ng. .lf'it is noti/desired to utilize the sleeve lt? as `a part .of 'mold for casting thebushing. the sleeve itl' maybe omitted from the present structure'since the voltage `.stress is distributed by the metal screen 27 in themauner previously described. From the foregoing description it is apparent that the present invention provides a bushing which may be cast Vfrom a thermoset resin. VSuch a bushing is suitable for lf .it is desired to utilize the bushing forY indoor Service. outdoor` service a ceramic Weather casing may be Apro-- vided on 'the exposed 'portion of the bushing. Bushings vconstructed in accordance with the present invention have been found to havefr less radio `interference than bushings of prior construction. r

ySincenumerous changes'may beV made in the` abovedescribed constructiomand dilerent `embodiments of .theY

invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing descriptionY or shown in the accompanying drawingV shall be interpreted as illustrative and not in a limitingsense.

.I `claim as my invention: 1

1. In an insulating bushing, in combinations cylindrical metal sleeve having a metal flange secured thereto, said Vsleeve having a plurality `of elongated longitudinal slots therein, a conductor extending longitudinally through and spaced from the sleeve, a thermoset epoxide vresin body lling the space between the conductor `and the sleeve and adhering to the inside of the sleeve, said slots permitting the sleeve to contract with the resin body, said resin body extending beyond the ends ofthe sleeve, and said conductorV extending beyond the ends of said resin body.

2. in an insulating bushing, in combination, a cylindrical metal sleeve having ametal flange secured thereto.

said sleeve having a plurality of elongated slots around its periphery a conductor 4extendinglongitudinally through and spaced from the sleeve, a thermoset resin body lil-ling the .space between the conductor land the sleeve `and ad hering to they inside of the sleeve, said slots permitting .the Vsleeve to contract with the resin body, a cylindriealrnetal screen embedded in the resin body concentrically with. :the

conductor, said resin body extending beyond the ends of the sleeve, 'and said conductor extending beyond the ends y of said resin body. p ,n Y

3. In Ian insulating bushing, in combination, a cylindrical metal sleeve having a metal flange secured thereto, said sleeve having a plurality of elongated longitudinal slots therein, a conductor extending longitudinally ythrough and spaced from the sleeve, a thermoset resin body filling the space between the conducto-r Iand the sleeve and adhering to the inside of the sleeve, said. slots permitting the sleeve to contract with theV resin body, said resin body extending beyond the ends of the sleeve, said conductor extending beyond the ends of said resin body, and apercelain casni-g disposed on said resin body above said flange.

4. In an insulating bushing, in. combination, Ya cylindrical metal sleeve having a metal flange secured thereto, a conductor extending longtiu'dinally through and spaced from the sleeve, a thermoset resin body lilling the. space between the conductor andthe sleeve and Yadheringto the inside of the slevee, said resin body 'extending beyond the ends of the sleeve, said conductor extending beyond*Y the ends of said resin body, a ceramic casing disposed on said resin body above said ilange, and a self-retainingilexible `and elastic material interposed between 'the casing and the resin body.

5. In .an insulating bushing, in combination, a cylindrical metal sleeve having a metal flange Asecured thereto, said sleeve having a plurality of elongated' longitudinal slots .therein,fa conductor extending longitudinally through and spaced from the sleeve, a thermoset resin body filling the space Vbetween the conductor `and the sleeve `an-d vadhering to the inside of the sleeve, sai-'d slots perrnittingthe sleeveV ytoV contract with the resin'body, said resin :body

extending beyond the ends of the sleeve, said-conductor extending beyond the ends of saidresin body, a porcelain resin body.

References Cited in the le of this patent UNITED STATES PATENTS Steinberger Apr. 8, 1924 6 Wirth .Tune 24, 1930 Janssen June 30, 1931 Dutton Oct. 8, 1957 FOREIGN PATENTS Great Britain Nov. 26, 1925 Farance May 19, 1954 Switzerland Mar. 31, 1958 

